Generically, a keyboard is a device made by a set of switches or keys, which generates an electrical signal when a mechanical pressure is applied in one of the keys. From this generic definition, a keyboard is any device based on pressure switches that can be placed in computers (desktop/laptop), pocket calculator, remote controls, mobile phones, music players (mp3, cd's, etc.), automated teller machine (ATM), electrical goods in general, sell machines (ex. Tobacco, drinks and food), elevators, ticket machines, laboratory equipments, electric and mechanical measurement equipments, tactile screens, interactive panels, etc.
A keyboard is a device composed by a set of sensors that when a mechanical pressure is applied in each one of them an electrical signal of binary type is generated, i.e., each sensor distinguishes only if there was a pressure applied or not on itself.
The keyboards are the devices more used as men-machine interaction systems. For most of the applications it is necessary to have keyboards which are light weight, of high reliability, insensible to electromagnetic interferences and with low production costs. Depending on the application, it can be necessary to follow aesthetic criteria, such as color, transparency, or presence or absence of moving parts and key sound.
When classified relatively to mechanical arrangement, there are basically two types of keyboards: the conventional ones based on pressure buttons and the laminar ones. In the conventional keyboards based on pressure buttons, each key is constituted basically by one button, one spring and one electrical contact. When a pressure is applied on the button, the electrical contact is shorted and the pressure is detected. The spring is used to put the button back in its initial position. This type of keyboard covers most of the computer keyboards. The laminar keyboards, also known as flexible membrane switches, normally presents a laminar construction with two sheets, being the electrical contacts constructed in each of them. The contacts are oppositely mounted one against the other in the two sheets, being separated normally by air. When a pressure is applied in a button, the upper sheet deforms itself and makes contact with the correspondent area of the lower sheet.
The conventional or the laminar keyboards with metallic contacts become less reliable after a long period of use. For instance, the electrical contact resistance when a key is pressed, ideally should be very low, but it tends to increase with time, due to accumulated dust, oxide formation in the contacts or to accidental spill of liquids over the keyboard. The increase of the resistance has as a consequence the incorrect readout from some keys. Besides that, in the laminar keyboards with metallic contacts, the membrane is deformed with each pressure in a key, leading to fatigue or even to rupture.
To eliminate these disadvantages from the keyboards using metallic contacts, some applications make use of Hall effect switches or capacitive keys. These types of switches present the disadvantages of needing a more complex geometry and electronic interface than the previous ones.
Another type of laminar keyboards uses piezoelectric materials. In this case, an applied pressure in the piezoelectric material produces a voltage variation in their terminals, which can be used to determine if there was pressure or not. Between the piezoelectric devices that can be used in keyboards manufacture, the crystals and the ceramics are the most used. These materials, besides giving good electrical signals, are rigid, break easily, are difficult to build in complex shapes and create electrical voltages which can be high when subjected to sound waves.
Recently laminar keyboards have been developed based on piezoelectric polymers. These resolve substantially the problems of the switches based on metallic contacts. They are also more advantageous in relation to the keyboards that use piezoelectric crystals or ceramics, since they are easier to process, more flexible and have a good mechanical response, being in this way the most suitable for this kind of applications.
Relatively to the readout circuits for the keys, the conventional keyboards based on pressure buttons and the laminar ones based on metallic contacts, usually just need coded circuits which output the correspondent binary code of the pressed key. Since each key conduct or interrupt the electric current, it is not necessary any kind of amplification circuit or voltage level adaptation. In keyboards based in piezoelectric polymers, normally the voltage or the current generated when a key is pressed is low. In this case and in case when it is necessary, there are some voltage or current amplification mechanism and or voltage level adaptation before the coded circuits. Usually these circuits are based in voltage or current amplifiers or voltage/current converters. Since the piezoelectric elements are sensible to sound waves, these circuits will amplify these same waves, giving wrong readouts from the keys. Despite the aforementioned facts, there are several applications, methods and geometries used as keyboards based in piezoelectric polymers.
The documents WO8102223, U.S. Pat. No. 4,328,441 and U.S. Pat. No. 4,633,123 use two films of piezoelectric polymers separated by an insulator material. In each of the piezoelectric films are impressed the metallic conductive contacts that forms the keys. The fact of possessing two piezoelectric films allows that the line and column of the local where the mechanic pressure is applied can be readout in an independent way: in this way the upper film reads only the line and the lower film reads only the column. In these applications are also described several readout electronic circuits from the keys.
The document U.S. Pat. No. 4,234,813 describes a keyboard based in piezoelectric polymers and uses their piezoelectric and pyroelectric proprieties to receive the stimulus. Like in the previous applications, two films of piezoelectric polymers are used: one to decode the lines and the other to decode the columns.
The documents U.S. Pat. No. 4,078,187 and U.S. Pat. No. 3,935,485 describe the use of a complex mechanic arrangement based in piezoelectric elements.
The document JP2103823 describes the use of several electrodes (two or three) of different dimensions and geometries for each key. Since the electrodes of different dimensions generate electrical signals of different amplitudes, it is possible to know what is the key pressed by means of the voltage generated in each contact. These electrodes are connected to the readout circuit by two or three conductive wires which connect one electrode of each key in parallel with the correspondent neighbor key.
The document U.S. Pat. No. 4,516,112 describes a transparent keyboard system. In this application there is not a clarification of the materials used in the electrodes in order to make them transparent.
Some piezoelectric polymeric materials respond also to the pyroelectric effect which produces a voltage proportional to temperature variation, as is mentioned in the document U.S. Pat. No. 4,975,616.